winnow

Winnowing removes lightweight debris from harvested grain by throwing the mix into the air and letting wind or a forced air current carry away the chaff, dust, and broken material while heavier kernels fall back down. It is one of the oldest and most reliable grain-cleaning operations, essential after threshing and before milling or storage.

The principle relies on terminal velocity: a grain kernel of 50 to 100 milligrams falls much faster than a piece of chaff weighing 1 to 10 milligrams. In traditional winnowing, a worker tosses grain from a basket into a breeze; the chaff drifts away while grain lands at the worker's feet. Industrial winnowers use a fan or air column to create consistent airflow, passing grain through a series of screens or falling streams. Some machines integrate the winnower with a thresher, producing clean grain in one pass. Others work as standalone units receiving already-threshed material.

Variants and limitations

Grain must have moisture content below roughly 14 percent or chaff will stick to kernels and resist separation. Long thin debris like straw tends to blow away more readily than short chaff or soil clods; stones and lumps will not separate by air alone and require screens or gravity tables downstream. Very small grains like millet or sesame need careful air velocity tuning to avoid over-winnowing and loss of the crop itself.

Modern combine harvesters do winnowing as part of their cleaning system, using air channels and sieves in tandem. Seed processors and mills often employ multi-stage winnowers with variable-speed fans to handle different grain types. The term comes from Old English and is cognate with words for wind; the physical action of tossing grain still visibly mimics the wind's sorting work.